1. Field of the Invention
The present invention mainly relates to an engine control system used in control of a rotational speed of a general-purpose engine having a small engine displacement, and particularly relates to an engine control system comprising: a governor lever which is oscillatably supported around an axis on an engine body, and connected to a throttle valve of a carburetor; a governor which increases a thrust for rotating the governor lever in a direction to close the throttle valve in accordance with an increase in an engine rotational speed; a control lever which is supported around an axis on a fixed structure, and operated to oscillate between a low-speed position and a high-speed position; a governor spring which is expandedly provided between the control lever and the governor lever, and which increases an urging force for rotating the governor lever in a direction to open the throttle valve; and an automatic choke device which is connected to a choke valve of the carburetor so as to automatically open and close the choke valve.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2006-242143 (specification of U.S. Pat. No. 7,246,794) discloses an engine control system comprising: a governor lever which is oscillatably supported around an axis on an engine body, and connected to a throttle valve of a carburetor; a governor which increases a thrust for rotating the governor lever in a direction to close the throttle valve in accordance with an increase in an engine rotational speed; and a governor spring which is expandedly provided between a fixed structure and the governor lever so as to urge the governor lever to rotate in a direction to open the throttle valve; wherein an automatic choke device is connected to a choke valve of an engine so as to automatically open and close the choke valve in accordance with increase and decrease in temperature of the engine.
In the conventional engine control system, a relatively large spring constant is imparted to the governor spring extendedly provided between the fixed structure and the governor lever in order to bring the engine into a high-speed operation state from start of the engine.
However, even if there is provided an automatic choke device, it is desirable that the engine rotational speed is set as low as possible from the start of the engine to the start of under-load operation of the engine in order to reduce the noise. From this point of view, there is a demand that a set load of the governor spring is arbitrarily switchable to a high level and a low level to make an engine rotational speed switchable to a high stage and a low stage.
In a general-purpose engine having a small engine displacement and required to be started at a low rotational speed, if an air/fuel ratio of an air/fuel mixture is set low (rich) by adjustment of the carburetor in order to obtain an excellent low-temperature starting performance of the engine, the air/fuel ratio becomes excessively low in high-temperature operation of the engine, leading to a disadvantage of an operational disorder or an increased fuel consumption.
Particularly in the system having an automatic choke device, a meticulous control is difficult to be conducted such that the air/fuel ratio is low at start of the engine, is slightly increased after the start, and is further increased at high temperatures of the engine.